Presentation on theme: "Chapter 2: Noise and Vibration By: Dr. Sara Yasina Yusuf | School of Environmental Engineering EAT 342: Noise Pollution Control."— Presentation transcript:
Chapter 2: Noise and Vibration By: Dr. Sara Yasina Yusuf | School of Environmental Engineering EAT 342: Noise Pollution Control
Page 2 Course Objectives & Learning Objectives This chapter reflects CO2: –Ability of defining the properties of sound, quantifying the noise levels and decibel as well as to characterize the noise. Learning Objectives for the chapter: –LO-1: Able to SKETCH pure tone wave and DEFINE the properties of sound waves (i.e. Frequency, amplitude, wavelength and period). CALCULATE the speed of sound, sound pressure, frequency and wavelength. –LO-2: DEFINE sound scale in decibel. –LO-3: DIFFERENTIATE and CALCULATE the Sound Power Level, Sound Intensity and Sound Pressure Level. –LO-4: CALCULATE the Sound Pressure/Power Level, summation of sound pressure using formula, table and graph, substraction and averaging of Sound Pressure/Power Level –LO-5: DISCUSS and ANALYZE noise characteristics, such as weighting networks, octave bands and rating systems –LO-6: DISCUSS the techniques in measuring community/environmental noises. COLLECT and ASSESS the L eq as well as L dn measured through the PBL assignment. –LO-7: DISCUSS various types of community noise sources and its criteria. COMPARE the measured data obtained via PBL and CRITIC on the levels to protect human health and welfare
Page 3 Content 1.The basic physics of sound –Sound and types of sound –Speed of sound –Sound pressure –Properties of Sound Waves –Frequency –Wavelength 2.Characteristics of noise and Desibel scale –Frequency and Loudness –Sound level and decibel scale –Weighting networks –Octave bands –Rating systems 3.Noise measurement 4.Community Noise Sources and Criteria –Transportation noise –Other internal combustion engines –Construction noise –Zoning and siting considerations –Levels to protect Health and Welfare
Page 4 1.The basic physics of sound – Sound & Types of Sound What is sound wave? A sound wave is an air pressure disturbance that results from vibration that propagates through an elastic medium (air, water, etc.) at a speed characteristic of that medium. Patterns of noise: Steady-state or continuous Intermittent Impulse or impact
Page 5 1.The basic physics of sound – Sound & Types of Sound Continuous noise is an uninterrupted sound level that varies less than 5 dB during the period of observation Intermittent noise is a continuous noise that persists for more than 1 second that is interrupted for more than 1 second Impulse noise is a change of sound pressure of 40 dB or more within 0.5 second with a duration of less than 1 second high pitch or intensity, lifetime of less than 1 sec.
Page 6 1.The basic physics of sound - Sound & Types of Sound Behaviour of sound waves Reflection Waves bounce off a surface Echoes & reverberation Refraction Waves bend when they pass through a boundary Diffraction Waves spread out when they pass through a small gap Interference 2 waves superpose to form a resultant wave of greater or lower amplitude
Page 7 1.The basic physics of sound – Sound & Types of Sound Reflection Interference
Page 8 1.The basic physics of sound – Sound & Types of Sound Example 2.0 A fishing boat received the echo 50 ms after sending it. The speed of sound in water is 1500 m/s Determine the depth of the water.
Page 9 1.The basic physics of sound – Speed of Sound In a free field, sound propagates with the velocity c defined by For the velocity of sound in air sufficiently accurate at normal temperatures, 0–30 o C where T K and T R are the temperature in Kelvin and Rankine, respectively where T C is the temperature in centigrade 0 F=R Eqn. 2.1 Eqn. 2.2 Eqn. 2.3
Page 10 1.The basic physics of sound – Speed of Sound Example 2.1 Determine the speed of sound at 20 o C (68 o F) in both metric (m/s) and English (ft/s) units. 0 F=R - 459
Page 11 1.The basic physics of sound – Sound Pressure For a pure tone, the sound pressure p can be described as where a is the amplitude in Pascals, ω is the angular frequency in radians per second, t is the time in seconds, and f is the frequency in hertz. Eqn. 2.4 Eqn. 2.5 A pure tone is a tone with a sinusoidal waveform, i.e. a sine or cosine wave
Page 12 1.The basic physics of sound – Sound Pressure Figure 2.1, and 2.2 show the pure tone oscillation, although pure tones do not often exist in nature. The field of acoustics and noise control has nearly uniformly adopted the metric system throughout and, as such, the unit used for measuring sound pressure is the Pascal(Pa).
Page 13 1.The basic physics of sound – Properties of Sound Waves Figure 2.1 : Pure tone
Page 14 1.The basic physics of sound – Properties of Sound Waves Figure 2.2: Pure tone from a tuning fork
Page 15 1.The basic physics of sound – Properties of Sound Waves The frequency of a sound indicates the number of cycles performed in 1 s: where T is the period of one full cycle. The unit for frequency is the hertz (Hz): A high frequency pure tone is perceived to have a high pitch. The audible frequency range to humans is 20–20,000 Hz Above 20,000 Hz is ultrasonic. Below 20 Hz is sometimes called infrasonic. Eqn. 2.6
Page 16 1.The basic physics of sound – Properties of Sound Waves Figure 2.3 : Frequency and amplitude
Page 17 1.The basic physics of sound – Properties of Sound Waves
Page 18 1.The basic physics of sound – Properties of Sound Waves The wavelength λ is equal to the distance the oscillations have propagated in the time period T: This shows that the wavelength is inversely proportional to the frequency. In the audio frequency range, the low frequencies have wavelengths of several meters (or feet), whereas the wavelengths for the high frequencies are only a few centimeters (or fractions of an inch). Eqn. 2.7
Page 19 1.The basic physics of sound – Properties of Sound Waves Figure 2.5 : Harmonic oscillation of pressure
Page 20 2.Characteristics of Noise – Frequency and Loudness There are two important characteristics of sound or noise - frequency and loudness. The number of pressure variations per second is called the frequency of sound, and is measured in Hertz (Hz) which is defined as cycles per second. The higher the frequency, the more high-pitched a sound is perceived. Frequency Pitch Amplitude Loudness
Page 21 2.Characteristics of Noise – Frequency and Loudness Example 2.2 Determine the wavelength of a 125-Hz and an 8000-Hz tone at 20 o C (68 o F) in both metric and English units.
Page 22 2.Characteristics of Noise – Frequency and Loudness The curves in Figure 2.6 indicates the loudness – the subjective interpretation of the magnitude of sound – for pure tones. The loudness of a sound depends on the wave's amplitude. The louder the sound, the higher the amplitude. So, amplitude is also a way of measuring the energy has. The higher the energy, the higher the amplitude resulting a louder sound.
Page 23 2.Characteristics of Noise – Frequency and Loudness Figure 2.6 : Normal equal loudness contours for pure tones. 20Hz
Page 24 2.Characteristics of Noise – Frequency and Loudness For example, as can be seen in Fig. 2.6, the threshold of hearing at 1000 Hz is about 4 dB (5 x Pa). A 20-Hz tone must have a sound pressure level about 70 dB higher than a tone at 1000 Hz in order for a person just to hear the tone. The curves also indicated the loudnessthe subjective interpretation of the magnitude of soundfor pure tones. The units describing loudness are called phons. By definition, the phon is equal to the sound pressure level (in dB) reference to 20 μPa of an equally loud 1000-Hz tone. Pain will occur when the loudness exceeds 120 phons.
Page 25 2.Characteristics of Noise – Sound Level and Decibel Scale As explained previously, sound is measured in unit of pressure and normally referred in Pascal and Newton per meter square or psi. The lowest audible sound pressure is Pa (or 20x10 -6 Pa equivalent 20 μPa). This value is very low as compared to sound emitted from a taking off jet i.e., 200 Pa. Note that the range of difference between these is so great. Therefore, a scale based on the logarithm of the ratios of the measured quantities is used. Measurements on this scale are called levels.
Page 26 2.Characteristics of Noise – Sound Level and Decibel Scale rms Sound Pressure Physically, the rms value is indicative of the energy density of the disturbance. Mathematically, the rms value is obtained by squaring the sound pressures at any instant of time and then integrating over the sample time and averaging the results. The rms value is then the square root of this time average: Where the overbar refers to the time-weighted average and T is the time period of the measurement Eqn. 2.8
Page 27 2.Characteristics of Noise – Sound Level and Decibel Scale Sound Power Travelling waves of sound pressure transmit energy in the direction of propagation of waves magnitude of the displacement times component of force in the direction of the displacement The rate at which this work is done Sound Power Level is defined as : Where, L w = Sound Power level in dB W = Sound Power in watt W o = Reference sound power = watt The standard reference power watt is the threshold power of our hearing. Eqn. 2.9 = WORK = (SOUND) POWER, Watt
Page 28 2.Characteristics of Noise – Sound Level and Decibel Scale Please note that, direct reading of sound power is not possible. Sound Level meter functions by measuring sound pressure or the difference in pressure due to vibration of air molecule, compared to 1 atm. Equation 2.2 describes the power emitted from a noise source. For example, when we speak, our voice vibrates the air molecule causing the pressure to increase and a sound power is generated. What is measured by the instrument is the pressure. Development of sound measurement instruments are currently based on the measurement of pressure and not the power.
Page 29 2.Characteristics of Noise – Sound Level and Decibel Scale Example 2.3 Given L w = 90 dB. What is the sound power in watt? W o = Reference sound power = watt Eqn. 2.10
Page 30 2.Characteristics of Noise – Sound Level and Decibel Scale Sound Intensity Sound intensity (I) is defined as the time-weighted average sound power per unit area normal to the direction of propagation of the sound wave I= Sound Intensity (watt/m 2 ) W = Sound Power (watt) A = area (sphere) normal to source (m 2 ) = 4πr 2, r is distance from source The higher the area, the lower will be the sound heard at a distance from origin. Therefore, sound intensity is reduced proportionately with increase in coverage area. Eqn. 2.11
Page 31 2.Characteristics of Noise – Sound Level and Decibel Scale Intensity is related to sound pressure in the following manner: Where, I = Intensity (watt/m 2 ) p rms = root mean square pressure, Pa ρ= density of medium (kg/m 3 ) c = speed of sound in medium (m/s) Eqn. 2.12
Page 32 2.Characteristics of Noise – Sound Level and Decibel Scale Both the density of air and speed of sound are a function of temperature. Given the temperature and the pressure, the density of air may be determined from Standard Table. Sound Intensity Level can be written as: Where, L I = Sound Intensity Level (dB) I = Sound Intensity (watt/m 2 ) Eqn. 2.13
Page 33 2.Characteristics of Noise – Sound Level and Decibel Scale Example 2.4 Given that a sound power in watt from a pile driver in a construction site is 1x10 -3 watt. Determine the sound intensity at the perimeter which will be heard by the following two cases: i) Heidi stands at a distance 15 meter from source ii) A hawker stands at a distance 50 meter from source Sound Intensity level? A = 4πr 2, r is distance from source
Page 34 2.Characteristics of Noise – Sound Level and Decibel Scale Sound Pressure Level In order to cope with the problem of an astronomical range of numbers of the sound pressure level (i.e; normal healthy level of Pa vs. Saturn rocket at liftoff of >200 Pa), a scale based on the logarithm is introduced as levels The unit for these types of measurement scales is the Bel, named after Alexander Graham Bell: L = levels, Bel Q = measured quantity Q 0 = reference quantity L = log 10 (Q/Q o ) unit: Bel Eqn. 2.14
Page 35 2.Characteristics of Noise – Sound Level and Decibel Scale The sound pressure level then is a logarithmic ratio L p defined as: where p rms = the sound pressure of interest (in Pa) and p ref = reference sound pressure (in Pa) usually chosen as the limit of hearing of 20 μPa. NOTE: (P log 10 x n ) = P (n) log 10 x The unit for the sound pressure level, SPL or L p, is the decibel (dB) Eqn. 2.15
Page 36 2.Characteristics of Noise – Sound Level and Decibel Scale The L P is measured against a standard reference pressure, p ref = p o = 2 x N/m 2 which is equivalent to zero decibels. The relationship between sound pressure and sound pressure level (with 20 μPa as the reference sound pressure) is shown in Table 2.1. A scale showing some common sound pressure level is shown in Figure 2.7.
Page 37 2.Characteristics of Noise – Sound Level and Decibel Scale Table 2.1
Page 38 2.Characteristics of Noise – Sound Level and Decibel Scale Figure 2.7: Relative scales of sound pressure levels
Page 39 2.Characteristics of Noise – Sound Level and Decibel Scale Example 2.5 Determine the sound pressure level for sound pressures of p = 1 Pa and p = 1 atm (1.013 × 10 5 Pa) (reference to 20 μPa)
Page 40 2.Characteristics of Noise – Sound Level and Decibel Scale Combining Sound Pressure Levels Since were dealing with the logarithmic heritage in SPL, adding the decibels is the same as multiplying them. For example, adding 0dB (20µPa) noise with 0dB to it, youll get a 6.02dB noise. Two approaches: 1) addition through formula, 2) addition through graphical solution
Page 41 2.Characteristics of Noise – Sound Level and Decibel Scale For skeptics, this can be demonstrated by converting the dB to SPL, adding them and converting back to dB. Thus, the addition of these Sound Pressure Level is denoted by: L p = 20 log 10 (P/P o )dB L pt = 10 log 10 [ Σ (10) Lpi/10 ]dB Eqn Eqn. 2.16
Page 42 2.Characteristics of Noise – Sound Level and Decibel Scale The addition of these Sound Power Level is denoted by: L w = 10 log 10 (w/w o )dB L wt = 10 log 10 [ Σ 10 (Lwi/10) ]dB Eqn Eqn. 2.18
Page 43 2.Characteristics of Noise – Sound Level and Decibel Scale A graphical solution for this type of problem is provided as in Figure 2.8. For noise pollution work, results should be reported to the nearest whole number.
Page 44 For equal decibel values, a shortcut method can be applied: 2.Characteristics of Noise – Sound Level and Decibel Scale n10 log 10 (n)n
Page 45 2.Characteristics of Noise – Sound Level and Decibel Scale Figure 2.8: Graph for solving decibel addition problems
Page 46 2.Characteristics of Noise – Sound Level and Decibel Scale Example 2.6 Three SPLs 68 db, 79 dB and 75 dB, what is SPL of combination? 68 dB 75 dB = dB 80.7dB 79 dB = 3.2
Page 47 2.Characteristics of Noise – Sound Level and Decibel Scale Alternatively, this can be solved by converting the readings to SPL, adding them and convert back to SPL:
Page 48 2.Characteristics of Noise – Sound Level and Decibel Scale Calculate the final sound power level that would be heard for noise levels 92, 98, 100, 95 and 85 dB using: I.Formula II.Graph
Page 49 2.Characteristics of Noise – Sound Level and Decibel Scale Averaging Sound Average sound can be calculated as the same as the calculation of summation of sound. As sound is referred in form of log, so the average requires calculation in form of pressure and power. The pressure is then calculated, averaged and finally antilog.
Page 50 2.Characteristics of Noise – Sound Level and Decibel Scale Average Sound Pressure Level is given as: Where, = Average Sound Pressure Level at reference pressure 20 μPa, dB (A) N = Number of sample L j = Sound Pressure level measured at reference pressure 20 μPa, dB(A) j= 1,2,3…n Eqn (a)
Page 51 2.Characteristics of Noise – Sound Level and Decibel Scale The latter equation is only applicable to sound levels in dBA It may also be used to compute average sound power levels if the factors of 20 are replaced with 10 s Where, = Average Sound Pressure Level at reference pressure 20 μPa, dB (A) N = Number of sample L j = Sound power level measured at reference power level W, dB(A) j= 1,2,3…n Eqn (b)
Page 52 2.Characteristics of Noise – Sound Level and Decibel Scale Example 2.7 Average the Sound Pressure Level for following field monitoring data, 51, 38, 78 and 68 dB(A). Eqn (a)
Page 53 2.Characteristics of Noise – Weighting Networks Weighting networks are used to account for the frequency of a sound. They are electronic filtering circuits built into the sound level meter to attenuate certain frequencies – with a prejudice something like that of the human ear. Normally, there are 3 weighting characteristics: A, B and C The very low frequencies are filtered quite severely by the A network, in a manner similar to the response of the ear, but only moderately by the B network and hardly at all by the C network. Therefore, if the measured sound level on the C network is much higher than that on the A network, much of the sound energy is concentrated in the low frequency region.
Page 54 2.Characteristics of Noise – Weighting Networks A Scale –Filters out low frequencies –Response curve is similar to sensitivity of human ear C Scale –Filters out very little (only the extreme low frequencies) –If a measurement is higher on the C scale than the A scale, the noise has a low frequency component –Used to estimate the effectiveness of ear protectors A specialized filter, the "D" weighting, has also been introduced for aircraft noise measurements. Figure 2.9 shows the response characteristics of the three basic networks as prescribed by the American National Standards Institute (ANSI) spec. no. S1.4 – 1971.
Page 55 2.Characteristics of Noise – Weighting Networks Figure 2.9: Frequency Response Characteristics of Various Weighting Networks
Page 56 2.Characteristics of Noise – Weighting Networks When a weighting network is used, the sound level meter is electronically subtracts or adds the number of dB shown at each frequency shown in Table 2.2 Readings taken when a network is in use are said to be sound levels rather than sound pressure levels. The readings taken are designated in decibels in one of the following forms: dB(A), dBa, dBA; dB(B), dBb, dBB; dB(C), dBc, dBC. Tabular notations may refer to L A, L B, L C
Page 57 2.Characteristics of Noise – Weighting Networks Table 2.2: Sound Level Meter network weighting values - CFA
Page 58 2.Characteristics of Noise – Weighting Networks Example 2.8 A new type 2 sound level meter is to be tested with two pure tone sources that emit 90 dB. The two sources are at 1,000 Hz and 100 Hz. Estimate the expected readings on the A, B and C weighting networks.
Page 59 2.Characteristics of Noise – Weighting Networks Example 2.9 The following sound levels were measured on the A, B, and C weighting networks: Source 1: 94 dB(A), 95 dB(B) and 96 dB(C) Source 2: 74 dB(A), 83 dB(B) and 90 dB(C) Characterize the sources as low frequency or mid/high frequency.
Page 60 Added Problem The measured octave band sound pressure levels around a punch press is given in table below. Determine the A-weighted sound level and the overall sound pressure level. Octave band center frequency, Hz LP (dB) CFA, dB LP + CFA (dBA) 2.Characteristics of Noise – Weighting Networks
Page 61 2.Characteristics of Noise – Octave Bands The human ear is sensitive to sound in the frequency range from approximately 16 Hz to 16 kHz. Impractical to measure the sound pressure level at each frequency in this range The measurements are made over an interval of frequency which is called the bandwidth and is specified by an upper and lower frequency limit f i+1 and f i are called cut-off frequencies. In acoustics the frequency bandwidths are usually specified in terms of octaves and one-third-octaves Normally, considering an 8 to 11 octave bands
Page 62 2.Characteristics of Noise – Octave Bands An octave is defined as an interval of frequency such that the upper frequency limit is twice the lower limit, that is: For the 1/3-octave bands, it is defined as: The center frequency of the band is defined as the geometric mean of the upper and lower frequencies for the interval: Relationship of center frequency for an octave? For 1/3 octave?
Page 63 Generation law for octave and third octave bands 2.Characteristics of Noise – Octave Bands Octave band oct. filter 1/3 Octave band third oct. filter
Page 65 Added Problem Find the geometric mean frequency for 1:1 octave and 1:3 octave bands for the following band no. 2.Characteristics of Noise – Octave Bands Octave Band1/3 Octave Band LowerCenterUpperLowerCenterUpper Frequency f 1 (Hz)f 0 (Hz)f 2 (Hz)f 1 (Hz)f 0 (Hz)f 2 (Hz)
Page 66 2.Characteristics of Noise – Octave Bands Figure 2.10: (a) One-third octave band analysis of a small electric motor. (b) Narrowband of analysis of a small electric motor
Page 67 2.Characteristics of Noise – Octave Bands Noise level measured with 1:1 Octave Band Filters
Page 68 2.Characteristics of Noise – Octave Bands Noise level measured with 1:3 Octave Band Filters
Page 69 2.Characteristics of Noise – Rating Systems Goals of Noise-Rating System –An ideal noise-rating system is one that allows measurements by sound level meters or analyzers to be summarized succinctly and yet represent noise exposure in a meaningful way –Our response to sound is strongly dependent on the frequency of the sound –Significant factors in annoyance: type of noise & time of day that it occurred –Ideal system: a) frequency, b) daytime or nighttime noise, c) capable of describing the cumulative noise exposure.
Page 70 2.Characteristics of Noise – Rating Systems The L N Concept L N is a statistical measure that indicates how frequently a particular sound level is exceeded. Example: if L 30 = 67 dB, means that 67 dB(A) was exceeded for 30% of the measuring time. A plot of against N (where N = 1%, 2%, 3%,….) look like the cumulative distribution curve (Figure 2.11) Allied to the cumulative distribution curve is the probability distribution curve (Figure 2.12) – showing how often the noise levels fall into certain class intervals.
Page 71 2.Characteristics of Noise – Rating Systems Figure 2.11: Cumulative distribution curve
Page 72 2.Characteristics of Noise – Rating Systems Figure 2.12: Probability distribution plot Frequency of occurrence, % Calculation of L 30 = 67 dB: Where; N = the sum of the percentages L = lower limit of the left-most class interval added Eqn. 2.21
Page 73 2.Characteristics of Noise – Rating Systems The L eq Concept The equivalent continuous equal energy level (L eq ) can be applied to any fluctuating noise level. It is expressed as: Eqn. 2.22
Page 74 Added Problem Refer to the attachment distributed in the class. Construct a cumulative distribution curve (Sound level vs. Percentage time greater than stated value) Find: a)L max b)L min c)L 1 d)L 50 e)L 90 f)L eq 2.Characteristics of Noise – Rating Systems
Page 75 2.Characteristics of Noise – Rating Systems Example 2.10 Consider the case where a noise level of 90 dBA exists for 10 minutes and is followed by a reduced noise level of 70 dBA for 30 minutes. What is the equivalent continuous equal energy level for the 40-minute period? Assume a five-minutes sampling interval.
Page 76 2.Characteristics of Noise – Rating Systems The L dn Concept L dn is the L eq computed over a 24-hr period with a penalty of 10 dBA for a designated night time period. Day-night average – subscript dn In airport noise applications, L dn is referred to LDN Night time period 10 pm to 7 am L dn equation is derived from the L eq equation with the time increment specified as 1 s (1 86, 400 seconds)
Page 77 2.Characteristics of Noise – Rating Systems So, eqn becomes: 10 log [1/86400] 49.4, the day-night average sound level: Eqn Eqn. 2.24
Page 78 2.Characteristics of Noise – Rating Systems Example 2.11 The USEPA estimated that, in 1974, the following was a typical noise exposure pattern for a factory worker living in an urban area. Estimate the L dn for the exposure shown. Time (h)Sound Level (dBA) 0000 – – – – – – – –
Page 80 Content 1.The basic physics of sound –Sound and types of sound –Speed of sound –Sound pressure –Properties of Sound Waves –Frequency –Wavelength 2.Characteristics of noise and Desibel scale –Frequency and Loudness –Sound level and decibel scale –Weighting networks –Octave bands –Rating systems 3.Noise measurement 4.Community Noise Sources and Criteria –Transportation noise –Other internal combustion engines –Construction noise –Zoning and siting considerations –Levels to protect Health and Welfare
Page 81 3.Noise Measurement – Measuring noise Noise measurement equipment depends on the task to be performed For an initial survey – a sound level meter (SLM) is adequate for a rapid evaluation and identification of potential problem areas To study and also determine the characteristics of a noise problem area – an SLM, frequency analyzer and recorder are needed
Page 82 3.Noise Measurement – Measuring noise –Sound Level Meter Used to measure the sound pressure level Available to cover a range of 20 to 180 dB Specifications refer to the American National Standards Institute (ANSI) – referred as Specifications for Sound Level Maters (ANSI S ) Weighting networks of A, B, C are provided – total loudness level for a particular situation with consideration of the sound frequency, intensity and impact levels. Weighting network A – most commonly used: discriminates against frequency below 500 Hz – encompasses the most sensitive hearing range. Measuring environmental noise should be supplemented by the time/duration to determine the total quantity of sound affecting people
Page 83 3.Noise Measurement – Measuring noise –Sound Level Meter (contd) Types of SLM: SLM provides setting for F (fast time response) and S (slow time response) Calibration by calibrator – generates a known decibel standard for QA/QC SLM TypeIntended Use 0laboratory reference standard 1for laboratory use, and for field use where the acoustical environment has to be closely specified and controlled 2suitable for general field applications 3primarily for field noise survey applications
Page 84 3.Noise Measurement – Measuring noise –Noise Dosimeter Measure the amount of potentially injurious noise to which an individual is exposed over a period of time Can be set to the desired level – total up the exposure time to noise above the set level Does not identify the noise sources To determine the noise exposure and culpability – dosimeter should be coupled with a frequency analyzer/ human observer to record noise source identities
Page 85 3.Noise Measurement – Measuring noise –Noise Dosimeter (contd) – Interpretation of results To calculate the noise exposure level of an employee working shifts of more or less than eight hours, it is necessary to normalise the employees exposure to an equivalent eight hour exposure (L Aeq,8h). where: L Aeq equals the equivalent continuous A-weighted sound pressure level occurring over the measured time; and T represents the shift length in hours (not to be confused with the sampling time). For shifts between 10 to 12 hours, add 1 dBA – extended shift In addition, shifts of 10 hours or more require adjustments to LAeq,8h values, as indicated in Table 3.1.
Page 86 3.Noise Measurement – Measuring noise Table 3.1 Correction factors for computing L Aeq,8h from L Aeq records
Page 87 Example 3.1 A personal noise dosimeter is placed on an employee for a representative period of six hours. At the end of the six hours, the L Aeq reading is 93 dB(A). The employee works a 10 hour shift. 3.Noise Measurement – Measuring noise
Page 88 –Sound Analyzer Frequency analyzer – measure complex sound and sound pressure according to frequency distribution. Supplemented with SLM Covers different frequency bands Example: octave band analyzer, impact noise analyzer (peak level and duration of impact noise) 3.Noise Measurement – Measuring noise
Page 89 –Cathode-Ray Oscillograph Observing the wave form of a noise and pattern Magnetic tape recorder makes possible the collection of noise information in the field and subsequent analysis of the data in the office or laboratory 3.Noise Measurement – Measuring noise
Page 90 –Background Noise Noise in the absence of the sound being measured that may contribute to and obscure the measured sound Correction can be made through subtraction method or application of correction factors (CF) as in Table Noise Measurement – Measuring noise ΔL, dBA Δ, dB ΔL, dBA Δ, dB Table 3.2: Background noise correction factor
Page 91 Example 3.2 The measured overall sound pressure level around a fan is 83 dB. The measured overall sound pressure level for the background (ambient) noise in the room where the fan is located is 77 dB. Determine the overall sound pressure level produced by the fan alone. 3.Noise Measurement – Measuring noise
Page 92 Added problem The experimental data shown below were measured around an air vent. The readings are the octave band sound pressure levels with the air flow stopped (background noise) and with the air flowing (data). Determine the octave band sound pressure levels and overall sound pressure level for the vent noise alone 3.Noise Measurement – Measuring noise Octave band center frequency, Hz Background L P, dB Data L P, dB
Page 93 Estimation of Community Reaction If noise spectrum data are not available, the L DN of the background noise, with suitable correctors may be used to estimate the anticipated community response to the environmental noise: The correction made to measured L dn accounted for the effect of annoyance due to several influencing factors (presented in Table 4.1) such as below: Nighttime Location Time of the year Previous noise exposure Average community reaction to noise based on L dn is given in Table Community Noise Sources and Criteria
Page 94 Estimation of Community Reaction – Table 4.1. Correctors to be Added to the Measured Day-Night Level for Various Influencing Factors for Community Noise Reaction b 4.Community Noise Sources and Criteria Influencing FactorDescription of conditionCF dn, dBA Noise SpectrumPure tones or impulsive noise present+5 No pure tone or impulsive sounds0 Type of locationQuiet suburban or rural community+10 Normal suburban community+5 Urban residential community0 Noisy urban residential community-5 Very noisy urban community-10 Time of yearSummer or year-round0 Winter only or windows always closed-5 Previous noise exposureNo prior experience with the intruding noise+5 Some prior experience with the noise or where the community is aware that good-faith efforts are being made to control noise 0 Considerable experience with the noise and the group associated with the source of noise has good community relations -5 Aware that the noise source is necessary, of limited duration, and/or an emergency situation -10 b Only one correction factor should be used from each category
Page 95 4.Community Noise Sources and Criteria Estimation of Community Reaction – Table 4.2. Average Community Reaction to Noise Based on the Day-Night Level, (L dn ) Corrected day-night level L dn (corrected) Expected community response <62 dBA (dn)No reaction dBA (dn)Complaints dBA (dn)Threats of community action >72 dBA (dn)Vigorous community action
Page 96 Example 4.1 The noise levels in a suburban area are given in Table 4.3. The area has had some prior experience with intrusive noises. There are no pure tone components of the noise, and it is not impulsive. The noise source will be present year-round. Determine the anticipated community response to the noise source. 4.Community Noise Sources and Criteria DurationA-weighted level Daytime 4 hours60 dBA 6 hours55 dBA 5 hours50 dBA Nighttime 2 hours45 dBA 7 hours40 dBA
Page 97 A.Aircraft Noise –The noise spectra of a wide fan jet (i.e. Boeing 747) reveal that sound pressure levels are higher on takeoff compared to landing –Smaller aircraft have lower sound pressure levels (except for turbojets) B.Highway Vehicle Noise –Predominant source of most automobiles during normal operation below about 55 km/h is the exhaust noise –At speed 80 km/h, tire noise is dominant source. For truck, at speed more than 80 km/h tire noise is dominant – the noisiest is cross- bar tread –Diesel trucks are 8 to 10 dB noisier than gasoline-powered –For motorcycle, dominant source of noise is exhaust – highly dependent on the speed –The U.S. Federal Highway Administration (FHA) has developed standards shown in Table Community Noise Sources and Criteria – Transportation noise
Page 98 4.Community Noise Sources and Criteria – Transportation noise Table 4.3. FHA noise standards for new construction a Either L eq or L 10 may be used, but not both. The levels are to be based on a 1-hour sample Land Use Category Exterior design noise level dBA a Description of land use category L eq L 10 A5760 Tracts of land in which serenity and quiet are of extraordinary significance and serve an important public need, and where the preservation of those qualities is essential if the area is to continue to serve its intended purpose. For example, such areas could include amphitheaters, particular parks or portions of parks, or open spaces, which are dedicated or recognized by appropriate local officials for activities requiring special qualities of serenity and quiet B6770 Residences, motels, hotels, public meeting rooms, schools, churches, libraries, hospitals, picnic areas, recreation areas, playgrounds, active sports areas, and parks C7275 Developed lands, properties, or activities, not included in categories A and B above DUnlimited Undeveloped lands E 52 (interior) 55 (interior) Public meeting rooms, school, churches, libraries, hospitals and other such public buildings
Page 99 These devices as listed in Table 4.4 are not significant to average residential noise levels in urban area However the relative annoyance of most of the equipment tends to be high – U.S. EPA, 1971 The 8-hour exposure level is in reference to the equipment operator 4.Community Noise Sources and Criteria – Other internal combustion engines Table 4.4. Summary of noise characteristics of internal combustion engines
Page common types of construction equipment – range of sound levels as in Table 4.5. Annoyance resulting from construction noise: –Single house construction in suburban communities will generate sporadic complaints if the boundary line 8-hour L eq exceeds 70 dBA –Major excavation and construction in a normal suburban community will generate threats of legal action if the boundary line 8-hour L eq exceeds 85 dBA 4.Community Noise Sources and Criteria – Construction noise Table 4.5. Range of sound levels from various type of construction equipment (based on limited available data samples. (Source: U. S. EPA, 1972)
Page 101 The U.S. Dept. of Housing and Urban Development (HUD) was charged with developing guides for zoning modifications or for siting of dwellings Annoyance for a specific noise exposure depended on both the average level of the noise and on the variability of the source of noise (Griffiths and Langdon, 1968) Table 4.6 list out criteria for new residential construction by HUD 4.Community Noise Sources and Criteria – Zoning and siting considerations General external exposuresAssessment Exceeds 89 dBA 60 minutes per 24 hours Unacceptable Exceeds 75 dBA 8 hours per 24 hours Exceeds 65 dBA 8 hours per 24 hours Discretionary: normally unacceptable Loud repititive sounds on site Does not exceed 65 dBA more than 8 hours per 24 hours Discretionary: normally acceptable Does not exceed 45 dBA more than 30 minutes per 24 hours Acceptable
Page 102 Noise criteria levels that is necessary to protect the health and welfare of U.S. citizens listed in Table Community Noise Sources and Criteria – Levels to protect Health and Welfare